always thicker at the bottom, and the stack is tapered. This type is not often used for power stations. Radial-brick chimneys may be 0.58 ft. thick at the thinnest part, as the brick is specially molded; the cross-section is always circular,
gO.6 H0.5 *0.4 gO.3
1.0 2.0 3.0 4.0 6.0 6.Q Uraff Inches, W.G.
FIG. 39.—Required horsepower for forced-draft fans.
1000 2000 3000 4000 5000 6000 7000 8000 Tip Speed, Ft.per Min.
FIG. 40.—Power and volume of forced-draft fans.
and tapers in the same manner as red-brick chimneys. Brick chimneys are the most durable of all the types. The firebrick portions of lining need be carried up only 30 ft. above the grates.
Steel chimneys have the advantages of lightness and strength, but since they are better conductors of heat, must be lined with brick for heights over 75 ft. except in forced-draft installations. They must be carefully inspected and painted from time to time, as they are subject to deterioration by corrosion.
Reinforced concrete is much stronger than brick and will stand high tensile strains like the steel chimney. The stack is therefore often built straight like the steel chimney, and is always considerably lighter than brick, as it may safely be much thinner. It is usually poured in 5-ft. or 6-ft. sections, which may be carried up a section a day, making erection rapid. No lining is required other than the short section of firebrick above the grates (30 ft.). It is one of the cheapest and most durable forms if well designed and built, but like all reinforced concrete, is dependent upon care and watchfulness during construction.
Foundations for brick chimneys are now made almost exclusively of concrete, and are designed on the basis of proper, bearing values, like any other foundation. They are usually spread or stepped out at the foot, in order to provide sufficient resistance to overturning from wind pressure.
COST OF BRICK STACKS
Approximate boiler horsepower Height Diameter Diameter square base outside Price
85 80 2 ft. 1 in. 7 ft. 5 in. 598.00
135 90 2 ft. 6 in. 8ft. 3 in. 786.00
200 100 2 ft. 11 in. 9 ft. 10 in. 1,226.00
300 110 3ft. Tin. 10ft. 2 in. 1,492.00
450 120 4 ft. 3 in. lift. 2 in. 1,785.00